This is true, the 6 GB RAM limit and four cores.
I run a pretty enterprise home lab, and I haven't ever seen the devices hit the resource limit.
I have around 3k IPS rules and TLS inspection for most categories of sites except the normal stuff like streaming, banking, etc that you'd not want or need to inspect.
For anyone it might help, I use these as inline proxies rather than as the gateway at the moment. So they have more than just internet traffic going through them, they also have segments of my LANs getting evaluated. Performance has been great so far.
I like OPN also. I've always appreciated the stability of the BSDs.
My only personal complaint with OPN/PF was the TLS inspection.
I've read about adding the modules to *Sense, but I haven't figured out the configuration pieces.
It just works with Sophos UTM and XG firewall, and the configuration was super easy.
You always use what you like though.